We propose to measure infrared fine structure lines of the
 elements Fe, Cl, F, P, and Ti in bright PDRs using the SWS
 and LWS in Fabry-Perot mode. This data can be used to derive the
 abundances of  these elements in the dense interstellar medium. ISO
 will be the only  instrument capable of studying depletions in this
 phase of the interstellar  mediumm in the foreseable future.